Compact source lamp having electrode construction providing arc stabilization



Jan. 16, 1968 J. WILSON 3,364,374

COMPACT SOURCE LAMP HAVING ELECTRODE CONSTRUCTION PROVIDING ARCSTABILIZATION Filed Sept. 28, 1964 Invervtov John WiLson b MW 8 HisAFFL'OT'TWG United States Patent COMPACT SOURCE LAMP HAVING ELEC- TRODECONSTRUCTION PROVEDING ARQ STABHLIZATEON John Wilson, Mayfield Heights,Ohio, assignor to General Electric Company, a corporation of New YorkFiled Sept. 28, 1964, Ser. No. 339,581 4 Claims. (Cl. 313-484) Thisinvention relates to a direct current compact source high pressure gasare lamp and is more particularly concerned with a construction of theelectrodes for stabilizing the arc.

A well-known form of compact source lamp is the high-pressure xenon arclamp wherein the arc is electrode-stabilized between electrodes whosedistance apart is less than the distance from the electrode axis to theenvelope wall. Such lamps are frequently used in optical systems wherethe light source must be small and precisely placed and this requiresthat the electrodes be accurately positioned. A problem which remains isthe tendency of the arc to bow or float upwards when the lamp isoperated horizontally. The arc instead of being axially centered, movesup toward the upper side of the anode. This means that the arc is offaxis and represents a form of instability. In the more extreme case, aportion of the are, sometimes referred to as a tail flame, projectsbeyond the upper side of the anode and may strike the bulb wall. Wherethe tail flame strikes the bulb wall, it may cause devitrification andas lamp life progresses, the wall becomes weakened and blows out,causing the lamp to fail. On account of the foregoing, high pressurecompact source gas are lamps for DC. operation have generally beenlimited to vertical or almost vertical operation, for instance i30 offvertical.

The use of a magnetic field to stabilize the electric discharge arc andcounter its tendency to bow upwards in horizontal operation is wellknown. A magnetic field may be produced by a permanent magnet or by anelectromagnet placed outside the discharge lamp. One may also use a coilwithin the lamp envelope and surrounding one of the electrodes toproduce an axial magnetic field. However magnetic stabilizationarrangements are generally inconvenient and restrict freedom in movingand aiming the lamp. A magnetic stabilizing coil within the lampenvelope tends to obstruct light from the arc and this may be anunacceptable drawback where the lamp is intended for an opticalprojection system.

The object of the invention is to provide a new and improved lampstructure, and particularly anode configluration, which will reduce oreliminate bowing of the arc and generation of a tail flame, therebyimproving stability of operation.

In accordance with my invention, I have found that in a compact sourcehigh pressure gas arc lamp for direct current operation, upward bowingor floating of the arc is greatly reduced and the generation of a tailflame sub stantially eliminated by changing the configuration of theanode from the conventional rounded or pointed tip to a flat tip or facenormal or transverse to the interelectrode axis and having a diameter ortransverse dimension at least 1.5 times the arc gap or interelectrodedistance. In

a preferred embodiment, the face diameter of the flattipped anode is inthe range of 1.75 to 2.5 times the arc gap under operating conditions.

For better appreciation of the invention and of its advantages,attention is now directed to the following description of a preferredembodiment to be read in conjunction with the accompanying drawing. Thefeatures of the invention believed to be novel will be more particularlypointed out in the appended claims.

Patented Jan. 16, 1968 In the drawing:

FIG. 1 is a side elevation illustrating a compact source are tubeembodying the invention.

FIG. 2 is a fragmentary view of. the arc gap region of a prior lampshowing the tail flame occurring with a rounded tip anode.

FIG. 3 is a view similar to FIG. 2 showing the effect of reducing thearc gap.

FIG. 4 is a side sectional view of a reflector lamp incorporating thearc tube of FIG. 1.

Referring to the drawing and more particularly to FIG. 1, theillustrated compact source are tube comprises a generally tubularenvelope 1 suitably made of quartz or quartz-like glass. The mid-sectionof the tube is bulbous and the ends are sealed by full diameter pinchseals 2, 3 through which extend respectively composite inleads 4, 5.Each inlead includes a thin intermediate foil portion 4a, 5a preferablyof molybdenum which makes the hermetic seal to the quartz, and an innerelectrode supporting portion indicated 4b, 5b suitably of tungsten wire.The outer and inner wire portions of the inleads are welded to theintermediate portion and the quartz of the seal is thickened or relievedover the welds, as indicated at 6, 7 for greater mechanical strength.

The cathode 8 is a thoriated tungsten cylindrical body having a pointedtip or conical nose 9 and mounted on inlead portion 412. The anode 10 isa massive generally cylindrical tungsten body mounted on inner leadportion 522. Heretofore, conventional anode design called for a roundedtip on the anode as indicated at 1141 in FIGS. 2 and 3.

Alternatively a conical tip was provided which became rounded off whenthe lamp was put in operation. The anode in accordance with my inventionis provided With a flat tip or face as shown at 11b in FIGS. 1 and 4.Where the flat tip does not extend across the entire transverse sectionor face of the anode, the shoulders are chamferred at 12. In theillustrated arc tube or lamp, the anode is heavy and additional supportis provided by a springy wire coil 13 having several smaller turnswrapped tightly around the anode body and a single expanded turn hearingagainst the envelope wall. The coil is located towards the rear of theanode body in order to avoid obstructing light from the are occurring inthe interelectrode gap. The are tube is exhausted and provided with asuitable ionizable filling, preferably xenon at a pressure of 1 or moreatmospheres, through an exhaust tube which leaves a tip 14 in theenvelope wall after tipping off.

In the lamp illustrated in FIG. 4, the arc tube is mounted within a sealzbeam type lamp housing formed of two pressed glass shells, at reflector15 and a cover or lens 16, peripherally sealed together. The are tube ismounted with its longitudinal or electrode axis coincident with theoptical axis of the reflector and the reflector is provided with arearwardly projecting neck portion 17 in which the cathode pinch seal isaccommodated. The cathode inlead 4 is connected to a metal thimble orferrule 18 fusion-sealed into the neck portion, and the anode inlead 5to a similar ferrule 19 in the center of lens 16, as per my copendingapplication Serial No. 336,187,. filed January 7, 1964, entitledReflector Arc Lamp, and assigned to the same assignee as thisapplication. Said application has now been abandoned in favor of mycontinuation-impart application Serial No. 602,436 filed October 31,1966, now Patent No. 3,341,731, issued September 12, 1967, similarlyentitled and assigned. By way of example, the arc tube overall lengthwas millimeters, cathode diameter 0.140", and anode diameter .250" (6.4mm.). The reflector enclosure or outer jacket is filled with nitrogen atabout /2 atmosphere through metal exhaust tube 21 which is shown tippedoff.

The illustrated arrangement, by accommodating cathode pinch seal 2 inneck portion 17 permits placing the arc gap of the arc tube at the focusof the reflector in a compact structure. Over the surface of revolutionformed by revolving the angle AOB about the optical axis ZZ', thereflector is elliptical with one focus of the ellipsoid located at thepoint 0. The surface of revolution ABAB is coated with alight-reflecting coating 22, either a metallic mirror film or a multiplelayer optical interference film, which focuses the light to the otherexternal or second focal point of the ellipse. The useful light from thearc is that which the reflector collects over the revolved angle AOBsubtended at the focus. The conical pointed cathode blocks only thelight which would proceed into the neck 17 of the reflector and bewasted anyway. An advantage fortuitously achieved by the invention isthat the flat-tipped anode with beveled shoulders 12 also interferesvery little with the useful light from the arc. Since the anode flap tipis oriented to face or look at the reflector, it blocks substantiallyonly that light which would spread out through the cover or lens 16 anddiverge uselessly. Thus the invention corrects upward bowing andinstability by means of a flat-tipped anode without appreciably reducingthe efliciency of the optical system.

FIG. 2 illustrates the kind of discharge which may occur with a compactsource direct current lamp on horizontal operation when using aconventional rounded tip anode 11a. The are gap in this instance was 2.2millimeters at the operating temperature. The plasma or luminous portionof the discharge indicated at 24 floats up towards the upper part of theanode and a tail flame 25 may extend obliquely beyond the anode andstrike the bulb wall. The upward floating and the tail flame appear tobe caused by convection effects. Where a tail flame strikes the bulbwall, devitrification may result and, as lamp life progresses, the wallnearest the tip of the tail flame is weakened and blows out causing thelamp to fail. With a floating or upwardly bowing arc, the tendency toflicker resulting from wandering of the are over the face of the anodeis increased.

The tendency of the arc to bow upwardly may be re duced by shorteningthe interelectrode gap. This is illustrated in FIG. 3 showing a lamputilizing a rounded tip anode 10 and an operating arc gap of 1.5millimeters between anode 8 and cathode 9. The are or plasma 26 is morenearly centered in the interelectrode gap and there is no tail flameextending beyond the upper side of the anode. However it is undesirableto go to a shorter gap because lumen output and lamp voltage arereduced. In a compact source lamp, the voltage is low while the currentis high and one of the limiting factors is the current carrying capacityof the electrode seals or inleads. Also when the current is increased,the reactive volt-ampere capacity of the ballast must likewise beincreased, and this is in the direction of greater cost and reducedefficiency. Therefore it is advantageous to keep the lamp voitage ashigh as possible in order to avoid increasing lamp current to make upfor reduced lumen output, and for this reason reduction of the arc gapis not desirable.

In accordance with my invention, are floating and upward bowing aresubstantially reduced and the tail flame eliminated by means of the flattip 11b on the anode as illustrated in FIGS. 1 and 4. FIG. 1 shows at 27the constrained or controlled are or luminous plasma obtained with theSOO-watt compact source xenon lamp illustrated when using an arc gap of2.3 mm. under operating conditions, and an anode flat tip portion 5.2mm. in diameter (dimension D). It is necessary to specify the gap underoperating conditions rather than at room temperature because theexpansion of the inleads is appreciable and operates to reduce the gapas the lamp warms up to operating temperature. In this case the gapwould be about 2.6 mm. if measured with the lamp cold. The foregoingcorresponds to a ratio of flat tip diameter to operating interelectrodegap of 2.25. As another example, with an interelectrode gap of 1.7 mm.under operating conditions, I have found a flat tip diameter of 3.6 mm.adequate; this corresponds to a flat tip diameter to interelectrode gapratio of 2.1. In both lamps, the diameter of the flat tip (dimension D)of the anode is large enough that the upper part of the arc plasma doesnot extend beyond the fiat portion of the anode face and the arc doesnot show the tendency to overlap the anode and project beyond as a tailflame.

I have found that with compact source gas are lamps of the instant kindwherein the current is in the range of 10 to 50 amperes, the minimumdiameter (transverse dimension D) of the fiat tip portion of the anodeshould be at least 1.5 times the arc gap or interelectrode distance,that is the distance along the axis of the lamp from the tip of thecathode to the face of the anode, under operating conditions. The longerthe interelectrode gap, the greater must be the diameter of the flatanode face in order to avoid floating and bowing of the arc. However forreasons of economy and also to avoid unnecessary obstruction of light,it is desirable to hold the anode down to the minimum size which willmeet the requirements of heat dissipation and arc stabilization bycontrol of upward bowing. I have found that these conflictingrequirements result in a preferred range for the diameter of the flatface portion of the anode from 1.75 to 2.5 times the operating arc gapof interelectrode distance. In the lamp illustrated in FIG. 4, when theratio of the diameter of the flat anode tip to the arc gap is within theabove preferred range, upward bowing or floating is substantiallyeliminated and the arc is stabilized without appreciable obstruction ofuseful light by the anode flat tip.

The specific embodiment of the invention which has been described indetail and illustrated is intended by way of example of the inventionWhose scope is to be determined by the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A compact source high pressure gas are lamp comprising a vitreousenvelope containing a filling of xenon at a pressure in excess of 1atmosphere and a pair of electrodes sealed into said envelope anddefining a short are gap whose length is less than the distance betweenthe electrode axis and the envelope wall, one of said electrodes beingan activated cathode having a pointed tip and the other of saidelectrodes being a more massive tungsten anode of generally cylindricalshape and having a flat tip whose minimum transverse dimension is atleast 1.5 times the interelectrode arc gap.

2. A compact source high pressure gas are lamp comprising a quartzenvelope containing a filling of Xenon at a pressure in excess of 1atmosphere, a pair of electrodes sealed into said envelope and defininga short arc gap whose length is less than the distance between theelectrode axis and the envelope wall, one of said electrodes being anactivated pointed tip cathode and the other of said electrodes being amassive generally cylindrical tungsten body axially disposed within theenvelope and having a flat tip at its forward face opposite the cathode,the diameter of the fiat portion of the anode face being in the range of1.75 to 2.5 times the interelectrode arc gap.

3. A compact source high pressure gas arc lamp comprising a quartzenvelope containing a filling of xenon at a pressure in excess of 1atmosphere, a pair of electrodes sealed into said envelope and defininga short are gap whose length is less than the distance between theelectrode axis and the envelope wall, one of said electrodes being anactivated pointed tip cathode of thoriated tungsten capable ofsupporting a current in the range of 10 to 50 amperes, and the other ofsaid electrodes being a massive generally cylindrical tungsten bodyaxially disposed within the envelope and having a flat tip at itsforward face opposite the cathode, the diameter of the flat portion ofthe anode face being in the range of 1.75 to 2.5 times theinterelectrode arc gap, no part of said anode projecting beyond saidflat portion toward the cathode.

4. A reflector lamp comprising a vitreous outer envelope including aconcave reflector, a compact source are tube mounted Within said outerenvelope with its longitudinal axis lying in the optical axis of thereflector, said are tube comprising a quartz envelope containing afilling of Xenon at a pressure in excess of 1 atmosphere, 3. pair ofelectrodes sealed therein and defining a short are gap Whose length isless than the distance from either electrode to the tube Wall, one ofsaid electrodes being an activated pointed tip cathode of thoriatedtungsten capable of supporting a current in the range of 10 to 50amperes, and the other of said electrodes being a massive generallycylindrical tungsten body axially disposed Within the envelope andhaving a flat tip at its forward face With chamferred shoulders, thediameter of the flat portion of the anode face being in the range of1.75 to 2.5 times the interelectrode gap, said anode face being orientedto look at the reflector in order to obstruct minimum light thereto andno part of said anode projecting beyond said flat portion toward thecathode.

References Cited UNITED STATES PATENTS 3,087,092 4/1963 Lafl'erty313-188 3,113,234 12/1963 Schlegez 313l84 X 3,114,855 12/1963 Story313217 X 3,138,731 6/1964 Beese 313-113 3,248,586 4/1966 Schlegel 313217X 3,248,591 4/1966 Arndt 313217 JAMES \V. LAWRENCE, Primary Examiner.STANLEY D. SOHLOS'SER, Examiner. S. A. SCHNEEBERGER, Assistant Examiner.

1. A COMPACT SOURCE HIGH PRESSURE GAS ARC LAMP COMPRISING A VITREOUSENVELOPE CONTAINING A FILLING OF XENON AT A PRESSURE IN EXCESS OF 1ATMOSPHERE AND A PAIR OF ELECTRODES SEALED INTO SAID ENVELOPE ANDDEFINING A SHORT ARC GAP WHOSE LENGTH IS LESS THAN THE DISTANCE BETWEENTHE ELECTRODE AXIS AND THE ENVELOPE WALL, ONE OF SAID ELECTRODES BEINGAN ACTIVATED CATHODE HAVING A POINTED TIP AND THE OTHER OF SAIDELECTRODES BEING A MORE MASSIVE TUNGSTEN ANODE OF GENERALLY CYLINDRICALSHAPE AND HAVING A FLAT TIP WHOSE MINIMUM TRANSVERSE DIMENSION IS ATLEAST 1.5 TIMES THE INTERELECTRODE ARC GAP.